Simplified apparatus for measuring primary electrical quantities



June 25, 1968 s. GEWIRTZ SIMPLIFIED APPARATUS FOR MEASURING PRIMARYELECTRICAL QUANTITIES Filed April 50, 1964 C c R N 20, SETER BIA 0cVOLTAGE 36A IV 20 a man SIB r '4 l0 0c 5' CURRENT 2 DIVIDER 28 8 4RESISTANCE 30C DIVIDER INVENTOR.

STANLEY GEW l RTZ United States Patent 3,390,329 SIMPLIFIED APPARATUSFOR MEASURING PRIMARY ELECTRICAL QUANTITIES Stanley Gewirtz, New York,N.Y., assignor to Solid State Systems, Inc., New York, N.Y.

Continuation-impart of application Ser. No. 182,456,

Mar. 26, 1962. This application Apr. 30, 1964, Ser.

6 Claims. (Cl. 324-115) The present application is acontinuation-in-part of my now abandoned application, Ser. No. 182,456,filed on Mar. 26, 1962 and entitled Simplified Means For MeasuringPrimary Electrical Quantities.

The present invention relates to the measurement of primary electricalquantities, such as current, voltage, resistance, capacitance, andinductance, and more particularly to simplified apparatus foraccomplishing the above described functions with a minimum ofcalibration.

It is the main object of the present invention to provide a novel,simple, reliable, and relatively inexpensive gang of components whichare collectively a multi-pu-rpose metering apparatus, and wherein aparticular feature of said gang is that it can be calibrated by means ofa single known constant which is externally located.

All presently available vacuum tube volt meters or amplifiedmultimetering devices comprise a series of variable resistors orrheostats which are located at either the input or output of a DCamplifier :and are individually switch-selected for each function of thecomponent circuit aggregate. These meters result in the cumulativeaddition of errors with the passage of time. These errors are usuallythe result of drift in the sensitivity of the DC amplifier and drift inthe impedance of the calibrating source resistors or variable resistanceelements.

Since the size and power requirements of portable measuring equipment,e.g., a vacuum tube volt meter (VTVM), a solid state volt meter (SSVM),or a transistorized portable laboratory (TPL), result in the utilizationof a DC amplifier with inherent drift, the present invention isconcerned with neutralizing both of the aforementioned sources oferrors.

It is therefore an object of the present invention to provide anapparatus for the measuring of primary electrical quantities which canbe externally calibrated to neutralize the drift in sensitivity of theDC amplifier employed in said apparatus.

It is another object of the present invention to provide potentiometersat the input of the DC amplifier of the measuring apparatus whicheliminates the addition of errors in the measuring apparatus with thepassage of time :and thereby eliminates the need for calibrating sourceresistors or variable resistance elements.

These other objects, features and additions of the present inventionwill become more apparent when considered in conjunction with thefollowing drawing which is a block diagram of the apparatus forming thepresent invention.

Referring now to the drawings, there is shown a gang of components whichcollectively comprise the multipurpose metering apparatus 10 of thepresent invention. The components include an AC to DC converter anddivider network 12, a DC voltage divider network 14, a DC currentdivider network 16, and a resistance divider network 18. The networks12, 14, 16 and 18 have a first input terminal 20, which is connected toground and is common to all of said networks, and a second inputterminal 22, 24, 26 and 28, respectively. The output of said networks12, 14, 16 and 18 have connected thereto one terminal of potentiometers30, 30A, 30B and 30C,

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respectively; said potentiometers having arms 31, 31A, 31B and 310,respectively, mechanically fixed thereon. The other terminals of saidpotentiometers are connected to ground by a common lead 3-2. The arms ofthe potentiometers are selectively connected to one of the inputterminals of the DC amplifier 34 by switching means 36 and lead 38, theother input terminal of said amplifier being connected to ground bymeans of the lead 40 connected thereto. The output of the DC amplifieris connected to a current meter, or similar electrical readout device,42 through a serially connected rheostat 44.

When an unknown DC voltage is to be measured, it is fed into the inputterminals 20 and 24 of the DC voltage divider network 14, apredetermined portion of the output thereof is taken from thepotentiometer 30A by means of the mechanically fixed arm 31A of said p0-tentiometer and is fed to the input of the DC amplifier 34 by means ofthe switch 36 which connects the input terminal of said amplifier tosaid arm 31A, such unknown DC voltage being amplified in said amplifier34 and its magnitude thence being indicated by the meter 42.

It is thus seen that the potentiometer 30A acts as a fixed ratio dividerto feed a predetermined portion of the voltage across said potentiometer30A to the input of the DC amplifier 34. This results from the fact thatthe arm 31A is mechanically fixed, and any change of the resistancevalue of the potentiometer, caused by the passage of time; i.e., by theaging of the potentiometer, does not change the ratio of resistancebetween the upper and lower arms of said potentiometer. Therefore, anychange in the resistance value of the potentiometer, with the passage oftime does not change the percentage of voltage taken from thepotentiometer by the fixed arm and fed to the DC amplifier. Thistherefore eliminates the errors normally caused by changes in impedanceof the calibrating resistance element with the passage of time. The sameresult is accomplished by the potentiometers 30, 30B and 30C, asconcerns the measurement of the primary electrical quantities associatedtherewith.

The error due to drift in the sensitivity of the DC amplifier isneutralized by the variable resistor 44. A DC source of calibratingpotential (not shown) is fed into the input terminals 20, 24 and themeter 42 is adjusted by means of the variable resistor 44 to indicate afull scale deflection, said full scale deflection corresponding to thevalue of the calibration potential.

It is thus seen that I have provided a system the accuracy of which islimited only by the accuracy of the calibrating potential and thelinearity of the DC amplifier, and which is easily calibrated by asingle variable resistor.

It should be noted that in present day multimeter devices, correction ofthe DC amplifier circuit to neutralize the drift in sensitivitynecessitates recalibration of the calibrating source resistors orvariable resistance elements. This operation requires that themultimeter be disassembled and reassembled and most often must beaccomplished with special equipment, said meter thereby being removedfrom use for an appreciable time.

While I have shown and described a preferred embodiment of my invention,it will be obvious to those skilled in the art that many modificationsand changes may be made therein without departing from the scope orspirit thereof as defined by the appended claims.

What is claimed is:

1. A circuit for measuring selectively a variety of primary electricalquantities, comprising a plurality of divider networks, a DC amplifier,a current measuring meter and means in series circuit with said meterand said amplifier for neutralizing drift in the sensitivity of saidamplifier, terminal means for applying different electrical quantitiesto said divider networks, a plurality of ratio voltage dividersrespectively connected to the outputs of said divider networks, andswitch means between the respective ratio voltage dividers and said DCamplifier for selectively connecting each of said divider networks tosaid amplifier, so that the meter measures an electrical quantityapplied to a selected network.

2. A circuit for measuring selectively a variety of primary electricalquantities, in accordance with claim 1 wherein, said means forneutralizing the drift in the sensitivity of the DC amplifier comprisesa rheostat.

3. A circuit for measuring selectively a variety of primary electricalquantities, in accordance with claim 2 wherein, said rheostat isexternally adjustable.

4. A circuit for measuring selectively a variety of primary electricalquantities, in accordance with claim 1 wherein, said divider networksare respectively an AC to DC converter and divider network, a DC voltagedivider, a DC current divider and a resistance divider.

5. A circuit for measuring selectively a variety of primary electricalquantities, in accordance with claim 1 wherein, said ratio voltagedividers comprise potentiometers having mechanically fixed arms, wherebythe voltage at the arms of said potentiometers are always a fixedpercentage of the voltage applied across said potentiometers andindependent of any change in the resistance value of saidpotentiometers.

6. A circuit for measuring selectively a variety of primary electricalquantities, in accordane with claim 4 wherein, said means forneutralizing the drift in the sensitivity of the DC amplifier comprisesa rheostat adapted to adjust the meter to indicate full scale deflectionwhen a known DC calibration potential is connected to the inputterminals of said DC voltage divider.

References Cited UNITED STATES PATENTS 6/1951 Triplett 324- X 9/1957Giuifrida 324l23 OTHER REFERENCES Voltmeter, War Department TechnicalManual TMll- 1239, pp. 1720.

1. A CIRCUIT FOR MEASURING SELECTIVELY A VARIETY OF PRIMARY ELECTRICALQUANTITIES, COMPRISING A PLURALITY OF DIVIDER NETWORKS, A DC AMPLIFIER,A CURRENT MEASURING METER AND MEANS IN SERIES CIRCUIT WITH SAID METERAND SAID AMPLIFIER FOR NEUTRALIZING DRIFT IN THE SENSITIVITY OF SAIDAMPLIFIER, TERMINAL MEANS FOR APPLYING DIFFERENT ELECTRICAL QUANTITIESTO SAID DIVIDER NETWORKS, A PLURALITY OF RATIO VOLTAGE DIVIDERSRESPECTIVELY CONNECTED TO THE OUTPUTS OF SAID DIVIDER NETWORKS, ANDSWITCH MEANS BETWEEN THE RESPECTIVE RATIO VOLTAGE DIVIDERS AND SAID DCAMPLIFIER FOR SELECTIVELY CONNECTING EACH OF SAID DIVIDER NETWORKS TOSAID AMPLIFIER, SO THAT THE METER MEASURES AN ELECTRICAL QUANTITYAPPLIED TO A SELECTED NETWORK.